Internal vibrational energy redistribution and vibrationally induced nonlinearity of HCN

Abstract

We explain internal vibrational energy redistribution (IVR) by the existence of concave regions of the potential energy surface (PES). Starting from some reported contradictions in HCN infrared band intensities and in rotational constants, we propose a changed assignment of the two bands at 16640 and 16674 cm −1. We assume a linear-to-bent structure transition of the HCN with high excited stretching modes. Due to the disappearance of the degeneracy of the bending mode of the linear structure, a zero energy part is regained which we add up the known experimental energy transition. The bent states emerge with a second rotational quantum number K at K=1 and K=0, and allow us to estimate an average geometry with an angle of 141° giving a snapshot of the isomerisation pathway up to the saddle point to CNH.